Tapped transformer winding



R. E. AYERs 3,113,281

TAPPED TRANsFoRMER WINDING 4 Sheets-Sheet 1 .No LOAD TAPS Dec. 3, 1963File'nec. 5, 1960 Dec. 3,

vFiled Dec. 5, 1960 R. E. AYERS TAPPED TRANSFORMER WINDING4Ysneets-sneet 2 Dec. 3, 1963 R. E. AYERs 3,113,281

TAPPED TRANSFORMER WINDING Filed Dec. 5, 1960 4 Sheets-Sheet 3 Dec. 3,1963 R. E. AYERs TAPPED TRANSFORMER WINDING 4 Sheets-Sheet 4 Filed Dec.5, 1960 b Jy Hfs MPa/75g.

United States Patent O 3,113,28i TAPPED TRANSFRMER WlNlNrG Ralph E.Ayers, Rome, Ga., assigner to General Eiectric Company, a corporation ofNew York Filled Dec. 5, i960, Ser. No. 73,748 6 Claims. (til. 336-150)This invention relates to electrical apparatus and more in particular toan improved winding yarrangement for regulating the output oftransformers.

Many types of electrical apparatus, such as transformers and the like,have a certain amount of flux, called leakage flux, which does not linkall the turns of every winding. The leakage iiux gives rise to a leakagereactance, which is a measure of the series inductance of the apparatuswinding and therefore is of importance when balancing a transformer witha load or another transformer. The leakage reactance of an untappedpower transformer is substantially constant throughout its operatingrange because the leakage flux travels primarily in a path comprising anunsaturable material (eg. air). Such untapped power transformers cantherefore be considered as hat reactance devices. However, it isgenerally desir-able to provide the windings of power transformers withregulating taps in order to provide a range of vol-tages that can beobtained from the apparatus. These tapped power transformers oftendisplay a severe change in transtormer reactance when tap connectionsare changed. This can be explained in part by the introduction of aleakage flux path perpendicular to the axis of the transformer core whenturns are added or removed along the length of the winding by changingtaps. There is also a change in the leakage flux which links less thanall of the turns of the winding when taps are changed, and a change inthe volts per turn in the windings.

One method of reducing the variations of leakage fiux in a tappedtransformer is to utilize the innermost winding adjacent the core forthe tapped winding. This arrangement is effective because the innermostwinding normally has the least amount of ux which does not link all ofthe turns and thereforeis of minimum leakage reactance. insulationconsiderations normally dictate that the outer winding of a powertransformer be the high voltage winding, and consequently prior art tapchanging transformers generally utilized taps on the innermost or lowvoltage winding to regulate output. -lowever, when the power handlingcapacity of the transformer is very large, the current in the lowvoltage winding becomes too great for it to be practical to switch tapsunder load. Consequently, the load tap changing connections were placedon the high voltage winding, and the high voltage winding was placed ina radially outside position for insulation purposes. This configurationresulted in the worst possibie arrangement as far as controllingvariations in leakage reactance is concerned.

Accordingly, it is 4an object of my invention to provide an improvedelectrical apparatus.

Another object of my invention is to provide an improved transformer,

A further object of my invention is to provide a winding arrangement fora tapped power transformer that minimizes reactance variations when tapsare changed under load.

VOther objects and advantages of the invention wiil become apparent froman examination of the specification, drawing, and claims which follow.

According to one aspect of my invention, fluctuations in reactance ofelectrical apparatus having a tapped winding can be reduced by splittinga high voltage coil into two serially connected sections and placing onesection adjacent a ground plane, such asa magnetic core, radiallyinwardly of a low voltage winding. The other high voltlll ICC

age coil is located radially outwardly of the low voltage winding, andthe load tap changing taps are placed in one of the high voltage coils.

ln the drawing:

FlGURE l is a schematic representation, partially in a cross-section, ofa transformer in accord with the teachings of my invention.

FIGURE 2 is a cross-sectional view of a transformer having the windingarrangement indicated in FIG. l.

yFlGURE 3 is an equivalent circuit for a modification of a transformer4winding in accord with my invention.

FGURE 4 is a schematic representation, laid out as a developed surface,iof the outer sec-tion of the high voltage winding in the modificationof FG. 3.

FlGURE 5 is a graph showing the improvement in reactance variationsachieved 1oy the practice of my invention.

My invention will now be explained by reference to the drawing. InFIGURES l and 2 a schematically illustrated transformer is seen tocomprise a core it) surrounded by winding coils il, d2, 4and 13. Thehigh voltage winding is formed from a hrst coil section il. and a secondcoil section l2. The low voltage winding coil i3 is sandwiched inbetween the first and second coil sections. The first high voltage coil11 is located in the radially innermost position adjacent the core i0,which will be a ground plane. The first coil section 11 may contain themajority of turns in the high voltage winding. The coil 1i may bedivided into two portions 23 and 29 having an equal number of turns. Theport-ion 23 may be connected by a lead 14 to a conventional switchingreactor i5. The portion 29 may he connected by 4a lead in to a reversingswitch i7, the operation of which will be explained in paragraphs thatfollow. The portions 23 and 29 each may comprise a plurality of disc orpancake-type coils serially connected together. Leads 27 from the highvoltage coil 11i. may be connected to line and/or to other similarwindings in conventional 4polyphase arrangements.

The outer 'nigh voltage coil i2 may be the regulating section of thewinding. The coil i2 has taps Si) which :can be connected under load ina buck or boost fashion to decrease or increase the number of activeturns in the high voltage windings. The coil 12 may be formed by placinga plurality of insulated conductor strands 2t) adjacent each other andspirally winding the adjacent strands in an axial layer along the core19. The exact number of adjacent conductor strands 2G in any specificcoil l2 will be dictated by the power capacity of the apparatus :and therange of regulation desired. This has been indicated by lnumbering theadjacent strands from i to N in the drawing. The specitic number oftimes each strand is wrapped around the `core will be determined by thenumber of turns necessary to produce the desired. regulation. Thevarious strands 2t) may be connested in series as indicated in FIG. l byconnecting the lower end of strand to the upper end of strand 2, and soon until each strand is connected to the ner/.t higher numbered strand.However, it will be appreciated by those skilled in the art that thestrands may be connected to obtain transpositions of various types inorder to achieve various electrical effects, such as impulsedistribution.

in order to achieve the maximum range of regulation from this type ofwinding arrangement, the strand i may be connected at tap G to a contact25 and the last strand N at the opposite end of the winding may have itstap R connected to a Contact 26. The reversing switch i7 is placedbetween the contacts 25 and 26 so that the turns in the wniding sectionl2 may be connected in either buck or boost fashion to the turns in thewinding section ii. A terminal 27' may be provided for connecting theswitching reactor to the reversing switch 17 in order to eliminate thewinding section 12 when desired.

When a transformer is of the type connected in a stepdown fashionbetween a high voltage transmission line and a low voltage distributingline, it is desirable to employ no-load taps 31 to enable thetransformer to be connected to diderent transmission lines operating ina range of different voltages. l have discovered that the 'no-load tapscan be interwound with the load tap changing taps of a transformer inaccord with the teachings of my invention. This can be accomplishedbecause standards set up by the power industry specify that the rio-loadtaps be 21/2% of the high Voltage rated connection voltage of atransformer, and the load tap changing taps be 11/4% of the high voltagerated connection Voltage. Thus there is a 2 to 1 ratio between thenumber of conductor turns necessary to produce the specified voltage inthe no-load taps and the turns required in the load taps. This enablesthe strands 2.0 in a high voltage coil 12 to be employed as load tapchanging taps 3@ by having a tap connection at each strand end. Thestrands 2@ can also be employed as no-load taps 31 by having a tapconnection at alternate strand ends.

The above-described type of interwinding arrangement of no-load taps andload taps has been illustrated in FIG- URES 3 and 4. FIGURE 3 shows anequivalent circuit for a transformer in accord with my invention havingload taps 3h and no-load taps 31 which may be inter- Wound in the outerhigh voltage coil 12, in the manner shown in FiG. 4. The no-load taps 31may be connected in series between one winding portion 29 and thereversing switch 17. Any conventional switching means 32 may be employedfor connecting to the desired no-load tap.

An outer regulating winding section 12. that can be connected to awinding 11 in the manner shown in FIG. 3 .is illustrated as a developedsurface in FG. 4. The winding section 12 may be formed, for example, byplacing a 'plurality of strands 2t) of insulated conductor adjacent eachother and winding them spirally into a coil that extends axially along acore (not shown in FlG. 4). Each strand is wrapped around the core thesame number of times, and this number is determined by the degree ofregulation desired and the power capacity of the apparatus. The load tapchanging taps 311 are obtained from 'terminals G, H, I, K, L, M, O, P,and Q at each strand end, and a terminal R at the end of the lowermoststrand. The various strands may be connected back to each other in themanner indicated in FIG. 1 with like lettered strands ends beingconnected to each other. The strands forming the loadtap changing taps3@ have been numbered 1 9 in order to correspond to the numbers used todesignate the strands in FG. 1. The no-load taps 31 are obtained fromterminals as indicated at E, C, B, D, A, and F, and strand ends havinglike letters would be electrically connected to each other. The strandsforming the no-load taps 31 have been numbered 11d-117. It will be ap--parent from FIG. 4 that each of the no-load taps 31 can produce avoltage variation twice as great as each load tap 3th because eachno-load tap is connected to twice as many turns.

It has been found that the space factor of the apparatus can beincreased if the load tap changing taps 31B have the ends of theirstrands Ztl starting on one side of the circumference of the winding 12and the no-load taps 31 have their strand ends starting on the oppositeside of the winding. This produces a saving in space because the bulkyconnections which must be brought out from the winding are located onopposite sides thereof and thus do not pile up on one another on thesame side of the `wind-ing. This arrangement has been indicated in FIG.4, since it is apparent that the load taps 39 and the noload taps 31would be approximately 180 apart if the winding were circular ratherthan a developed surface as illustrated.

lt will be apparent to those .skilled in the art that a tapped windingas illustrated in FIGURE 4 employs substantialiy the entire axial lengthof the core for active ampere-turns. The result is. that unbalances inampereturns are minimized because any gaps that exist in the regulatingwinding when turns are not employed are distributed throughout theentire length of the winding rather than being grouped together asadjacent turns. Thus, the effect of the inactive' turns on ampere-turnunbalance is less than in winding arrangements where the inactive turnsare adjacent to each other and thus produce socalied holes in thewinding.

FTGURE 5 is a graph showing the desirable affect of my invention onreactance fluctuations in commercial transform rs. The curves l and ilin the graph represent the reactance of transformers at various tappositions and are plotted in terms of the tap position reactance as apercentage of the reactance of the transformer at its rated connectionvoltage, the later reactance being taken as The curve 1 is an average ofthe reactance of five, three-phase, class 15 kv. primary connectionvoltage transformers having a rating of 2000 kva., constructed in themanner practiced by the prior art. The curve Il is the reactance of one,three-phase, class 15 kv. primary connection voltage transformer havinga rating of 2000 lava., made in accord with the teachings of myinvention. 1t should be noted that the prior art transformers had anaverage reactance fluctuation of about 20% from the maximum buoi;position (161s) to the maximum boost position (16K). However, thetransformer in accord with my invention showed reactance variations ofless than plus or minus 5% between these two extreme positions.

The following theory is believed to be an explanation for the desirablereduction in reactance variation of the transformer constructed inaccord with rny invention. Reactance variations are believed to becaused by changes in leakage flux resulting from an unbalance inampereturns, and also from changes in the number of volts per turnproduced by the apparatus. In a voltage stepdown arrangement, when theoutput of the low voltage winding is raised by a change in -tappositions, high voltage iwinding turns are bucked. ln this situation thelow voltage winding 13 and the tapped section 12 of the high voltagewinding act together in opposition to the other section 11 of the highvoltage winding because their ampereturns are in the same direction.This tends to raise the reactance between the high and low voltagewindings. At the same time, the increased volts per turn of the highvoltage and low voltage winding tends :to decrease the reactance. Thus,these two effects tend to work against each other to minimize reactancevariations. On the other hand, when the output of the low voltagewinding is lowered by a change in taps, the high voltage winding turnsare boosted and the windings are in the so-called form-S design. Thismeans that the inner and outer sections 11 and 12 of the high voltagewinding have their ampere-turns in the same direction and are opposedIto the ampere-turns of the low voltage |winding. This arrangement tendsto lower the reactance between the high and low voltage windings. At thesame time, the decreased volts per turn of the high and low voltagewindings tends to increase reactance. It is thus apparent that my novelwinding arrangement produces phenomena that tend to oppose each other sofar as the reactance produced is concerned. Consequently, reactancefluctuations caused by tap changes under load are kept to a minimum.

1t will be understood, of course, that while `the forms of the inventionherein shown and described constitute the preferred embodiments thereof,it is no-t intended herein to illustrate all the possible equivalent4forms or ramifications of `the invention. lt will also be understoodthat the words used are words o-f description rather than of limitation,and that various changes may be made Without departing from the spiritor scope of the invention herein disclosed, land it is aimed in theappended claims to cover all such changes as fall within the true spiritand scope of the invention.

What I claim as new and desi-re to secure by Letters Patent of theUnited States is:

1. In a transformer, the combination comprising a magnetic core, aplurality of windings around said core, said windings comprising la pairof serially-connected high voltage coils and a low voltage coil, one ofsaid high voltage coils being located adjacent said core, said lowvoltage coil surrounding said one high voltage coil radially outwardlythereof, the other of said high voltage coils surrounding said lowvoltage coil nadially outwardly thereof, and the radially outermost ofsaid high voltage coils being tapped at a plurality of locations toprovide terminals for changing the output of said transformer underload.

2. In a transformer, the combination comprising a magnetic core, aplurality of windings around said core, said windings comprising a pairof serially-connected high voltage coils and a low voltage coil, one ofsaid high voltage coils being located adjacent said core, said lowvoltage coil surrounding said one high voltage coil radially outwardlythereof, the other of said high voltage coils surrounding said lowvoltage coil radially outwardly thereof, the radially outermost of saidhigh voltage coils being tapped at a plurality of locations to provideterminals vfor changing the output of said transformer under load, andsaid youtermost high voltage coil comprising a plurality of axiallyadjacen-t insulated conductor strands spirally wound in an axialdirection so as to form la layer of interleaved lturns.

3. A power transformer having a rated connection voltage, saidtransforme-r comprising a magnetic core having a leg, a plurality ofwindings around said core leg, said windings comprising a pair ofserially-connected high voltage coils and a low voltage coil, one ofsaid high volt age coils surrounding said core leg, said low voltagecoil surrounding said one high voltage coil radially outwardly thereof,the other of said high voltage coils surrounding said low voltage coilradially outwardly thereof, the radially outermost high voltage coilbeing tapped at a plurality of locations to provide terminals forchanging the output of said transformer under load, said outermost highvoltage coil comprising a plurality of axially adjacent insulatedconductor strands spirally wound in an axial direction so as to form alayer of interleaved turns, and each of said strands being wound to form1a number of turns surrounding said core leg that produces a voltage ofapproximately 114% of the rated connection voltage of said transformer.

4. A power transformer, having a rated connection voltage, saidtransformer comprising a magnetic core having a leg, a plurality ofwindings around said core leg, said windings comprising a pair ofserially-connected high voltage coils and a low voltage coil, one ofsaid high voltage coils surrounding said core leg, said low voltage coilsurrounding said one high voltage coil radially outwardly thereof, theother of said high voltage coils surrounding said low voltage coilradially outwardly thereof, the radially outermost of said high voltagecoils being tapped at Ia plurality of locations to provide terminals forchanging the output of said transformer under load, said outermost highvoltage coil comprising a plurality of axially adjacent insulatedconductor strands spirally Wound in an axial direction so as to form alayer of interleaved turns, each of said strands :being wound to yform anumber of turns around said core leg that produces a voltage ofapproximately 111% of the rated connection voltage of said transformer,and some of said strands being serially connected to form pairs ofstrands having a number off turns between tap connections sufficient toproduce 2.1/2 of the rated connection voltage of said transforme-r, thelast mentioned tap connections providing no-load taps for saidtransformer.

5. A transformer as recited in claim 4 in which the terminals of saidno-load taps are on the circumference of said outermost high voltagecoil on the opposite side thereof Ifrom the terminals of the `load. tapchanging taps.

6. In a transformer, a high voltage winding containing load tap changingtaps and no-load taps, said Winding comprising a plurality of axiallyadjacent strands of insulated conductor material -spirally coiled in anaxial direction to form a layer of turns, each strand being coiled thesame number of times, ends of strands at one end of said winding beingconnected to ends of strands at the opposite end of said winding toproduce series connected turns, and tap connections for said no-loadtaps bein-g connected between twice as many series connected strands assaid load tap changing taps, whereby there is a 2 to 1 ratio between thenumber of turns between load tap changing taps and the number of turnsbetween no-load taps.

References Cited in the tile of this patent UNITED STATES PATENTS1,761,732 Kochling June 3, 1930 2,710,947 Gaston lune 14, 1955 2,757,347Pozaryski July 31, 1956 2,840,790l Vogel June 24, 1958 2,922,132 KreuzerIan. 19, 1960

1. IN A TRANSFORMER, THE COMBINATION COMPRISING A MAGNETIC CORE, APLURALITY OF WINDINGS AROUND SAID CORE, SAID WINDINGS COMPRISING A PAIROF SERIALLY-CONNECTED HIGH VOLTAGE COILS AND A LOW VOLTAGE COIL, ONE OFSAID HIGH VOLTAGE COILS BEING LOCATED ADJACENT SAID CORE, SAID LOWVOLTAGE COIL SURROUNDING SAID ONE HIGH VOLTAGE COIL RADIALLY OUTWARDLYTHEREOF, THE OTHER OF SAID HIGH VOLTAGE COILS SURROUNDING SAID LOWVOLTAGE COIL RADIALLY OUTWARDLY THEREOF,